The present disclosure relates to, for example, a protective circuit and a charging device including the protective circuit in regard to charging and discharging of a secondary battery.
As a charging device with respect to a secondary battery, for example, a lithium ion secondary battery, a charging device in which charging output is prohibited when charging voltage is excessive is suggested. For example, in Japanese Unexamined Patent Application Publication No. 2007-28802, the following is described. That is, if it is detected that a battery voltage reaches a predetermined overcharging protective voltage, a charging control transistor is turned off. In addition, in Japanese Unexamined Patent Application Publication No. 2007-28802, when charging voltage is decreased to charging restarting voltage, the charging control transistor is turned on.
An example of a protective circuit in the related art is illustrated in FIG. 10. When a P-channel type field effect transistor (hereinafter, appropriately referred to as “FET”) Q12 is turned on, charging power is supplied to a secondary battery BT. A charging control portion 21 observes charging voltage between the FET Q12 and the secondary battery BT via resistance, and compares the charging voltage and a preset threshold value (overcharging protective voltage). According to the comparison result, a charging output permitting signal is output from the charging control portion 21. For example, the charging control portion 21 includes a microcomputer.
When the charging voltage is smaller than the threshold value, it is determined to be a normal charging state. Thus, a high level of charging output permitting signal is output from the charging control portion 21. An N-channel type FET Q11 is turned on by the charging output permitting signal. A drain of the FET Q11 is connected to a gate of the FET Q12, due to the fact that the FET Q11 is turned on, the FET Q12 is turned on, the charging power is supplied to the battery BT, and the battery BT is charged.
On the other hand, when the charging voltage is the threshold value or more, it is determined to be an abnormal charging state, and the charging control portion 21 outputs a low level of charging output stopping signal. Thereby, the FET Q11 is turned off and the FET Q12 is turned off. As a result, the charging power is not supplied to the battery BT and the charging is stopped.
FIG. 11 illustrates another example of the charging control circuit in the related art. Similarly to the configuration of FIG. 10, the charging control portion 21 observes charging voltage, when the charging voltage is smaller than the threshold value, it is determined to be a normal charging state, and a high level of charging output permitting signal is output from the charging control portion 21. The FET Q11 and the FET Q12 are turned on by the charging output permitting signal.
In addition, a protective IC (Integrated Circuit) 22 is installed, and the charging voltage is observed by the protective IC 22. If the charging voltage is the threshold value or more, the protective IC 22 outputs a high level of charging stopping signal. The charging stopping signal is supplied to a gate of an N-channel type FET Q13, and the FET Q13 is turned on. Since a drain of the FET Q13 is connected to the gate of the FET Q11 and the source is connected to ground (a negative electrode of the battery BT), if the FET Q13 is turned on, the FET Q11 is turned off. Therefore, the FET Q12 is turned off and the charging is stopped.